Simultaneous measurements of inertial acceleration and flow structures during coarse-grain entrainment under steady currents

Abstract

An instrumented smart sediment particle (SSP) is used to investigate the entrainment of spherical coarse grains resting on a rough bed comprising semi-spherical elements. The SSP contains electronic sensors facilitating the direct measurement of tri-axial accelerations. The instantaneous flow field is measured simultaneously using two-dimensional particle tracking velocimetry (PTV), enabling joint analysis of accelerations and flow structures during entrainment. Two different initial placement positions of the SSP are tested. Both particle vibration and complete entrainment events are observed in experiments. Results show larger maximum acceleration and longer duration of motion with more significant obstruction by surrounding bed roughness, which entails greater momentum transfer at the initiation of entrainment. During vibration events, acceleration can occasionally reach the threshold value but with no entrainment because of the low persistence of relevant flow features inducing energy transfer. Further analysis on instantaneous flow structures indicates that, at the threshold flow condition, the occurrence and persistence of the “sweep-outward interaction” quadrant motion sequence (i.e. Q4-Q1 sequence) in front the particle, combined with the transient increase of vorticity due to the passage of clockwise vortices above the particle, is the key to energy transfer and complete entrainment. The Q4-Q1 sequence may either be caused by the lower half of anti-clockwise vortices or occur independently. Generally, the joint analysis of acceleration and flow structures provides a new perspective to studying coarse grain entrainment, especially a potential to relate near-bed flow structures to the status of particle motion in the future.